1. Field of Invention
This invention relates to poly(alkylene arylates), such as poly(ethylene terephthalate), PET; poly(propylene terephthalate), PPT; poly(butylene terephthalate), PBT; poly(ethylene naphthalate), PEN; poly(propylene napthalate), PPN; poly(butylene naphthalate); poly(ethylene isophthalate), PEI; poly(propylene isophthalate), PPI; poly(butylene isophthalate), PBI; homopolymers and their copolymers and mixtures, containing the residue of organic titanate-ligand catalyst systems. The poly(alkylene arylate)s possess better optical properties than similar polymers heretofore made with other organic titanate-ligand catalysts. Resulting PET, for example, is particularly useful in preparing transparent articles, such as films, that have excellent clarity, reduced light scattering and absorb less light than conventional PET. Thus, PET resins made with the catalyst have particular utility as the substrate for x-ray and photographic films.
2. Description of Related Art
Poly(ethylene terephthalate), PET, is a widely used polyester typically manufactured by two routes: (1) transesterification of a dialkyl terephthalate ester (e.g., dimethyl terephthalate) with ethylene glycol to form an intermediate bis-2-hydroxyethyl terephthalate, followed by polycondensation to form the PET; or, (2) by direct esterification of terephthalic acid with ethylene glycol, followed by polycondensation to form PET. A catalyst is commonly used to speed the reaction in either case. The same or different catalyst may be selected for the transesterification and polycondensation steps.
Many commercial processes use manganese or zinc salts as the catalyst for the transesterification step. Antimony, in the form of a glycol solution of antimony oxide, typically is used as the polycondensation catalyst in either the transesterification or direct esterification processes outlined above. There is an interest in replacing antimony with another catalyst, however, since insoluble antimony species tend to be formed which increase the polymer darkness, scatter light, and interfere with spinning or other forming. Furthermore, antimony catalysts have come under increasing regulatory pressure. Thus, there is a need for new polycondensation catalysts that reduce or replace antimony in the manufacture of PET and other poly(alkylene arylate)s.
Organic titanates, such as tetraisopropyl and tetra n-butyltitanates, are known to be effective polycondensation catalysts for preparing poly(alkylene arylates) in general, and frequently are the catalyst of choice in the manufacture of polybutylene terephthalate (PBT) because of their higher reactivity than conventional antimony catalysts. Organic titanates are not generally used in the manufacture of PET, however, because residual titanate catalyst tends to react with trace impurities formed during the polycondensation and processing of PET (e.g., aldehydes), generating yellow discoloration that cannot be tolerated in products typically fabricated from PET (e.g., x-ray and photographic films, bottles, and packaging film).
Lack of glycol solubility also is a practical limitation for most organic titanate catalysts. It is preferred to add catalyst to a continuous polycondensation reaction as a dilute glycol solution (rather than a dispersion) to obtain uniform distribution of the small quantities of catalyst that are employed. Organic titanates typically form a precipitate when added to a glycol, which tends to complicate manufacturing control and introduces product quality problems due to non-uniform distribution of catalyst in the reaction mass.
Numerous binary compositions containing organic titanates and phosphorus compounds (organic and inorganic) have been proposed in the technical and patent literature for use as a polycondensation catalyst in the manufacture of poly(alkylene arylates). For example, it has been proposed to add phosphoric acid, or other phosphorus based compounds, along with organic titanates to control color by complexing the residual titanate catalyst. The use of such strong complexing agents, however, invariably reduces efficiency of the titanate catalyst and introduces polymerization control problems. Thus, there is an ongoing need for a non-antimony based polycondensation catalyst that is glycol-soluble, efficient, and produces poly(alkylene arylates) in general, and PET and PPT in particular, having excellent optical properties.